<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <title>层级关系</title>
    </head>
    <body>
        <ul>
            <li style="width: 100px;height: 100px;z-index: 2;float: left;position: absolute;left: 0;">1</li>
            <li style="width: 300px;height: 300px;z-index: -1;float: left;position: absolute;left: 0;">2</li>
            <li style="width: 200px;height: 200px;z-index: 1;float: left;position: absolute;left: 0;">3</li>
        </ul>
    </body>
</html>